While the invention is subject to a wide range of applications, it is especially suitable for use in processing pre-recorded digital audio tapes and with digital audio tape recording equipment that requires an anti-copying feature.
Very high fidelity digital audio tape (DAT) recording techniques have been recently developed. However, such tape devices present a severe problem for organizations engaged in manufacturing and/or marketing of pre-recorded music and other program material.
The recordings made by such consumer type DAT recorders are of such high quality that it is difficult, if not impossible, for listeners to differentiate between the original recorded tapes and copies. This causes serious problems for firms producing or marketing pre-recorded tapes or compact disks for sale to the public because the sale of a single tape or compact disk can lead to literally hundreds of unauthorized copies.
CBS Laboratories has suggested a solution to the problem. CBS proposed that the program material be specially processed so as to identify the pre-recorded material. This identification would cause consumer tape recorders to be switched off.
The CBS method provides identification by eliminating a narrow band of speech and music components. In other words, a hole is introduced in the spectrum. Unfortunately, such spectrum holes are, under certain conditions, audible. Indeed, Kahn U.S. Pat. No. 4,217,661 discloses a method for eliminating cuts or holes in the frequency response of telephone line frequency extenders. In laboratory tests, conducted by Kahn Communications, Inc., of Carle Place, N.Y., on such equipment, a significant improvement was noted in the sound quality of voice and music signals transmitted through such equipment. Thus, it is apparent that listeners can hear cuts or holes in frequency response.